1. Field of the Invention
The present invention relates to a method for recycling an excess of a recyclable cold curing aqueous coating composition after applying on a substrate. Particularly, the present invention relates to a method for recycling said coating composition which may be collected with a collecting solution in a coating booth and then filtered, concentrated and separated into a concentrated coating solution and a separated solution, and consequently recycled after readjusting them, respectively.
2. Description of Related Art
Aqueous coating compositions having corrosion resistance are typically used as a primer for parts of an automobile, industrial machines, agricultural machinery and the like. When the aqueous coating compositions are applied on a workpiece which has poor heat resistance or has high heat capacity, it has to be dried and cured at a temperature of below 80.degree. C. They are generally called as cold curing aqueous coating compositions.
When the cold curing aqueous coating compositions are applied by spray coating in a coating booth, much amount of paint dust which is not deposited on a substrate is present. The spray dust is dissolved and dispersed into a booth aqueous solution in the coating booth to be collected. The amount of the collected dust is very large, and it is very difficult to dispose of waste paint in view of environmental pollution.
Many efforts to recycle the paint dust collected in the booth aqueous solution have been made, and for example a recycle method is proposed in Japanese Patent Kokai Publication Sho. 49(1974)-51324. The Japanese Publication discloses that sprayed dust of aqueous coating composition is collected by booth aqueous solution to obtain a diluted solution of the coating composition obtained diluted solution is filtered though a reverse osmosis membrane or ultrafiltration membrane to collect and concentrate the spray dust. Then, the concentrated coating composition is adjusted to as high as the non-volatile content of the original components in aqueous coating composition and subjected to reapplication. The "non-volatile" in this context are non-volatile components component including film-forming aqueous resin (such as an aqueous alkyd resin and an aqueous acrylic resin), pigments and the like.
The method described in the Publication, however, has a problem that when the diluted dust solution collected by the booth aqueous solution is filtered and concentrated by ultrafiltration, an aqueous solution separated from the diluted dust solution by filtering is produced in a very large amount. The collected coating composition can be reused, but the separated aqueous solution should be discarded. Therefore, it is also proposed that the separated aqueous solution is returned into the coating booth and recycled as booth aqueous solution.
However, the concentrated coating composition and the aqueous solution separated from the diluted aqueous solution have many disadvantages when repeatedly recycled. For example, in the concentrated coating composition, the concentration of volatile components reduces and the amount of neutralized amine also reduces, and therefore, the pH of the coating composition gradually decreases. In the separated aqueous solution, the concentration of low molecular component separated from the coating solution, for example, a residual monomer and a by-product which are contained in a main binder of the coating composition, or decomposed product generated from the resin during neutralizing and storing the resinous varnish, increases in every recycling use. If the above concentrated coating composition and the aqueous solution continue to be recycled, the hydration stability of the resin against the change in both solvent composition, and pH, which influences recycling characteristics, decreases, and at last the coating composition may aggregate or separate because of the decrease of the stability for of the resin contained in the coating composition.
In order to solve the above disadvantages, it is proposed that the hydrophilic property of the resin contained in the original aqueous coating composition increases so as to enhance the hydration stability before hydration of the resin in recycle use. On applying the above method, however, the coating composition may show defects in film performance, such as salt spray resistance, water resistance and the like. The cold curing coating compositions having corrosion resistance have been known, but there has been no cold curing coating composition which overcomes the above disadvantages and has a recycling property. In addition, according to the above method, the advantages which occur in recycling the aqueous solution after being filtered and separated as a booth aqueous solution are not improved.